Frost growth or icing on heat exchangers adversely affects performance of refrigeration systems by blocking the air flow path and increasing thermal resistance. In a mobile air conditioning system such as found on a motor vehicle, an inappropriate (a) placement of a thermistor or (b) compressor cycling strategy could cause the evaporator temperature to drop below freezing point which leads to icing on evaporator surfaces. Icing on the outside heat exchanger of a heat pump system, where the inner fluid temperature could operate well below 0 degrees C. during the heating mode, is particularly concerning. As the ice builds up, the heat pump has to cease heating operation and switch to de-icing/defrosting mode, during which a separate source is required to continue supplying heat. In severe scenarios, a de-icing cycle could consume over 20% of the time in the heating mode.
Control strategies are a common approach to mitigate or suppress icing. Prior control strategies include minimizing the potential between evaporating temperature and air inlet temperature (see, for example, U.S. Published Patent Application 2015/0107278) or elevating heat pressure to allow operation at lower ambient temperature (see, for example, U.S. Published Patent Application 2006/0288716). The effectiveness of such control strategies and the impact on system performance remain unclear due to lack of testing data. In addition, extra temperature and pressure sensors are generally required in order to implement the strategies.
Additive coating is another popular approach to avoid or delay icing and reduce the icing cycle/energy. Additive coating aims to create a hydrophobic (or hydrophilic in a less popular fashion) structure on the fin surface of the heat exchanger that facilitates condensate shedding and eventually delays icing. For example, Takasawa et al. (U.S. Published Patent Application 2014/0231052) proposed a pretreated fin material with a hydrophobic film on one surface and a hydrophilic film on the other. During condensation, a water droplet can be quickly removed by bringing it into contact with the hydrophilic film. Consequently, a favorable heat exchanger function can be maintained without any increase in ventilation resistance. In this approach, the cross linked hydrophobic film is resin based and formed by baking the fins after the application of hydrophobic coating.
Oligornetic silane compositions containing, for example, methyltrimethoxysilane, are disclosed in the U.S. Pat. No. 6,451,382 B2 to coat new or used heat exchange apparatus via dipping or spraying for heat transfer improvement and corrosion prevention. The chemical formula of the silane is Rn1Si(OR2)4-n, where R1 presents a C1-C6 alkyl group, a C6-C8 aryly group or a functional group including at least one of vinyl, acrylic, amino, mercapto, or vinyl chloride functional groups, R2 represents a C1-C6 alkyl or acetyl group and n is a number of 1 or 2.
A self-healing, slippery liquid-infused porous surface (SLIPS) is described in the U.S. Pat. No. 9,121,306 B2, where a lubricant fluid that has a chemical affinity to the substrate is used to wet the substrate. The lubricant on the SLIPS sticks to the surface. The adhesion of the lubricant to the surface is greater than the adhesion of water to the surface. With time, the lubricant can be drained away from the surface. Based on the inventors' search, among the existing coating methods, (a) lithography is not easy to scale, (b) dip coating or spray coating results in a thick layer and tends to decrease heat transfer and (c) liquid infusion is not durable due to lubricant loss. This document relates to a new and improved method of forming a superhydrophobic layer on a heat exchanger housing. That method is relatively simple, economical, reliable, durable and scalable.